But, in fact as many as ten per cent of bombs may have failed to explode – which means over 500 bombs could be lying dormant yet to be discovered.

While many have been discovered over the years, usually while building work is being carried out and sites excavated – it is likely there are still hundreds out there.

The device was identified as a WWII "two pounder" shell (pictured: Sgt Riva, of the bomb disposal unit, holds the shell after detonating it on the Downs)

These unexploded bombs still present a risk to construction projects taking place across the city. The devices often entered the ground unnoticed at high speed and penetrated several meters beneath the ground, where they are vulnerable to disturbances from building work.

After the air raid "all clear" sounded and it was safe to emerge from your shelter, one of the greatest dangers was from unexploded bombs. The one pictured here was nicknamed Satan and landed in Beckington Road, Knowle, during the raid of January 3, 1941.

If one gets hit, it could take up to 72 hours for the detonator to activate.

The information compiled by Zetica includes records from central government, the Ministry of Defence, and the German Luftwaffe and is the biggest database of its kind in the UK.

Some recent Bristol bomb discoveries

In the last month , three unexploded bombs have been found in the Bristol Channel close to Hinkley Point nuclear power station.

Dave Chapman, believed he had hit a stone when renovating his garden and continued trying to dig it up, hitting it repeatedly.

It took the EOD team around six hours to remove the ammunition from the residential street.

The area, including Muller Road where Mr Chapman found the shell was a target for Luftwaffe due to its close proximity to the Royal Artillery battery on Purdown where anti-aircraft guns tried to shoot down German aircrafts.